Infrared sensor for soil or water and method of operation thereof

ABSTRACT

An infrared (IR) sensor and a method of detecting molecular species in a liquid. In one embodiment, the method comprises i) generating IR light from an IR light source; ii) receiving in an optical fiber the IR light from the IR light source, wherein a selective ion-exchange (IX) medium is associated with the optical fiber and the IR light generates an evanescent field about the optical fiber as the IR light propagates therethrough, the selective IX medium configured to transport an ion species in a subject liquid about the optical fiber; and iii) receiving in an IR light detector the IR light from the optical fiber, wherein the ion species affects the evanescent field and thereby a characteristic of the IR light.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. Non-Provisional patentapplication Ser. No. 15/990,195, filed May 25, 2018, which claims thebenefit, under 35 U.S.C. § 119(e), of the filing of U.S. ProvisionalPatent Application Ser. No. 62/512,147, filed May 29, 2017, entitled“Optical Sensor for Nutrient and Contaminant Detection in AgriculturalSoils and Environmental Waters,” commonly assigned with this applicationand incorporated herein by reference.

TECHNICAL FIELD

This application is directed, in general, to chemical sensors and, morespecifically, to an infrared (IR) sensor capable of detecting molecularspecies, and thereby the presence of nutrients or contaminants inagricultural soils or environmental and industrial waters and a methodof detecting molecular species in a liquid.

BACKGROUND

Nitrate sensors are becoming increasingly important tools for water andsoil quality monitoring and resource management. Ultraviolet (UV)absorption sensors are today's standard for water quality analysis(Pellerin, et al., “Optical Techniques for the Determination of Nitratein Environmental Waters: Guidelines for Instrument Selection, Operation,Deployment, Maintenance, Quality Assurance, and Data Reporting: U.S.Geological Survey Techniques and Methods,” USGS, Vol. 1-D5 (2013), and,Sah, “Nitrate-nitrogen Determination—A Critical Review,” Commun. SoilSci. Plant Anal., vol. 25, pp. 2841-2869 (1994)).

Unfortunately, UV sensors capable of delivering continuous data duringprolonged deployment periods not only cost between $15,000 and $25,000,but are also vulnerable to interference from inorganic and organicsubstances that absorb light at wavelengths similar to those of nitrate.These substances include nitrite, bromide and chromophoric dissolvedorganic carbon (DOC), which are common in both water and soil.

SUMMARY

One aspect provides an IR sensor and a method of detecting molecularspecies in a liquid. In one embodiment, the IR sensor includes: (1) anIR light source configured to emit IR light, (2) a sensing elementconfigured to receive the IR light, the IR light generating anevanescent field about the sensing element as the IR light propagatestherethrough, molecules in a subject liquid interacting with theevanescent field and affecting a characteristic of the IR light and (3)an IR light detector configured to receive the IR light from the sensingelement and detect the characteristic.

In another embodiment, the IR sensor includes: (1) an IR light source,(2) an optical fiber configured to receive IR light from the IR lightsource, (3) a selective ion-exchange (IX) medium associated with theoptical fiber, the IR light generating an evanescent field about theoptical fiber as the IR light propagates therethrough, the selective IXmedium configured to transport an ion species in a subject liquid aboutthe optical fiber and (4) an IR light detector configured to receive theIR light from the optical fiber, the ion species affecting theevanescent field and thereby a characteristic of the IR light.

Another aspect provides a method of detecting molecular species in aliquid. In one embodiment, the method includes: (1) propagating IR lightthrough a sensing element, the propagating generating an evanescentfield about the sensing element, molecules in a subject liquid proximatethe sensing element interacting with and affecting the evanescent field;and (2) detecting a characteristic of the IR light affected by theevanescent field.

Another aspect provides a method of operating an infrared (IR) sensor,comprising: i) generating IR light from an IR light source; ii)receiving in an optical fiber the IR light from the IR light source,wherein a selective ion-exchange (IX) medium is associated with theoptical fiber and the IR light generates an evanescent field about theoptical fiber as the IR light propagates therethrough, the selective IXmedium configured to transport an ion species in a subject liquid aboutthe optical fiber; and iii) receiving in an IR light detector the IRlight from the optical fiber, wherein the ion species affects theevanescent field and thereby a characteristic of the IR light.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now made to the following descriptions taken in conjunctionwith the accompanying drawings, in which:

FIG. 1A schematically shows a selective IX medium with NO₃ anions andquaternary ammonium cations shown as N⁺R₃;

FIG. 1B schematically shows how the selective IX medium of FIG. 1Afunctions;

FIG. 2 schematically shows a cross-section of one embodiment of an IRsensor employing an optical fiber and an ion-exchange coating as theselective IX medium;

FIG. 3 schematically shows a cross-section of one embodiment of an IRsensor employing an optical fiber and a permselective membrane as theselective IX medium;

FIG. 4 schematically shows IR detection using the IR sensor;

FIG. 5 is a cross-sectional view of one embodiment of an IR sensorconfigured for analyzing water;

FIG. 6 is a cross-sectional view of one embodiment of an IR sensorembodiment configured for analyzing soil;

FIG. 7A is a partial isometric view of one embodiment of concentricfilter meshes for filtering soil;

FIG. 7B is an isometric view of concentric filter meshes installed insoil;

FIG. 8 is a cross-sectional schematic illustration of a test apparatusfor demonstrating the feasibility of an IR sensor constructed accordingto the description herein;

FIG. 9A is a graph illustrating ATR measurements of nitrate dissolved inwater (at concentrations of 12.5 ppm to 200 ppm) and data obtained froma solution extracted from soil;

FIG. 9B is a graph illustrating absorbance peaks as a function of NO₃concentration;

FIG. 10 is a graph illustrating ATR measurements of nitrate dissolved inwater and absorbed in an IX resin for 30 minutes; and

FIGS. 11A-11D are graphs illustrating ATR measurements of nitratedissolved in water and absorbed in an IX membrane at concentrations of12 ppm, 25 ppm, 50 ppm and 100 ppm, respectively;

FIG. 12 is a graph illustrating the evolution of NO₃ peak areasrepresented in the graphs of FIGS. 11A-11D as a function of time; and

FIG. 13 is a flow diagram of one embodiment of a method of IR detectingmolecular species in a liquid.

DETAILED DESCRIPTION

As stated above, nitrate sensors are becoming increasingly importanttools for water and soil quality monitoring and resource management. Inwater, excess nutrients, particularly nitrogen and phosphorus, cantrigger algal blooms and biodiversity loss with consequences that affectthe economy and pose a threat to human health. The cost of freshwaternutrient pollution in the United States alone has been estimated atUS$2.2 billion per year (Dodds, et al., “Eutrophication of U.S.Freshwaters: Analysis of Potential Economic Damages,” Environ. Sci.Technol., vol. 43, pp. 12-19 (2009)).

In agriculture, nitrogen fixing in soils accounts for over 60% ofglobal, water-soluble fertilizer market share, but approximately 30% ofthe fertilizer applied to North American soils is wasted due tooverapplication and runoff. With U.S. farmers spending US$12 billionannually on fertilizer, nitrate sensors could be used to reducefertilizer consumption and cost substantially, as well as the labor andfuel costs associated with fertilizer application.

As stated above, while UV absorption sensors are today's standard forwater quality analysis, they are expensive and vulnerable tointerference from common inorganic and organic substances. Theseshortcomings have significantly limited their application and use.

It is realized herein that components that function in the IR spectralrange may be combined to form an IR sensor. Such components may becommercially available at little cost. Many of the embodiments aresuitable to detect nutrients or contaminants in either soils or waters,which makes them particularly attractive for agricultural, environmentaland residential or industrial wastewater treatment use. Given theirpotentially low cost, the embodiments are particularly amenable forforming sensor networks, in which each sensor transmits its data to acentral site for storage and likely further analysis. Indeed, suchnetworks may be capable of delivering new kinds of analysis and tacticaland strategic services to farmers, conservationists, operators of waterworks and public safety or national security agencies.

It is more specifically realized herein that a commercially availableselective IX medium (e.g., an IX resin coating or permselectivemembrane) may be used in conjunction with a sensing element (e.g., anoptical fiber or coated wafer) to yield an IR sensor which, in manyembodiments, has a range of sensitivity from 0.001 ppm to 100 ppm, andbeyond, at a price far lower than conventional UV sensors.

Embodiments of the IR sensor illustrated and described herein employselective IX medium appropriate for sensing nitrate. However, thoseskilled in the pertinent art will, with the benefit of this disclosure,understand that alternative or additional selective IX media may beemployed in the sensor to allow it to sense corresponding alternative oradditional nutrients (e.g., nitrogen, phosphorus, sulfate, or potassium)or contaminants (e.g., arsenic).

FIG. 1A schematically shows a selective IX medium 100 by itself, withNO₃ anions (e.g., an NO₃ anion 110) and quaternary ammonium cationsshown as N⁺R₃ (e.g., an N⁺R₃ cation 120).

Assuming the selective IX medium 100 to be an anion-exchange membrane,FIG. 1B schematically shows how the anion-exchange membrane 100 of FIG.1A functions. First, a definition is in order. “Subject liquid” isdefined herein as a liquid that the IR sensor has measured, is measuringor will be measuring. In real-world applications, the subject liquidwill usually be water containing dissolved nutrients or contaminants. Inthe context of environmental waters or residential or industrialwastewaters, the subject liquid may be the waters themselves or thewaters following some particulate filtration. In the context ofagricultural soils, the subject liquid is typically water drained fromor filtered through the soil.

Given this definition, a subject liquid (e.g., environmental water orsoil drainage), perhaps containing NO₃ anions 110 and N⁺R₃ cations 120,is placed in contact with one side of the anion-exchange membrane 100(e.g., the right-hand side of the anion-exchange membrane 100 as FIG. 1Bis oriented). The net negative charge of the NO₃ anion 110 attracts itto the anion-exchange membrane 100, causing the NO₃ anion 110 to absorb,and perhaps pass entirely through, it. In contrast, the net positivecharge of the N⁺R₃ cation 120 repels it from the anion-exchange membrane100, dissuading the N⁺R₃ cation 120 from absorbing or passing throughit. A cation-exchange membrane acts in the opposite manner, attractingand absorbing cations and repelling anions.

FIG. 2 schematically shows a cross-section of one embodiment of an IRsensor 200 employing an optical fiber 210 and an IX coating 220 as theselective IX medium 100 of FIG. 1B. An IR source 230, which may includea commercially-available IR laser diode, quantum cascade laser (QCL) orthermal emitter, emits IR light 240, which is directed into an end ofthe optical fiber 210. The optical fiber 210, which may be acommercially-available optical fiber, serves as a waveguide for the IRlight 240. Accordingly, the IR light 240 reflects off a boundary surfaceof the optical fiber 210 as it propagates therethrough. A sawtooth lineshown in the optical fiber 210 represents this propagation andreflection in FIG. 2. As those skilled in the pertinent art understand,the propagation and reflection of the IR light 240 through the opticalfiber 210 creates an evanescent field (not shown in FIG. 2) surroundingthe optical fiber 210.

An IR detector 250, which may include a commercially-available thermalor photonic detector, receives the propagated IR light 240 from an endof the optical fiber 110 and produces an electrical signal as a functionthereof. The IR detector 250 may include signal processing hardware,perhaps operating under the control of software or firmware, to receiveand process the electrical signal in some manner. In many embodiments,the electrical signal is analyzed to determine amplitude attenuationindicating IR light absorption occurring as a function of molecularspecies (e.g., ion) concentration proximate the sensing element (theoptical fiber 210 in the embodiment of FIG. 2). The IR detector 250 mayfurther include communication circuitry configured to transmit theelectrical signal or a processed form of the electrical signal, perhapsin digital form and perhaps for remote, centralized collection andanalysis.

In various alternative embodiments, the resin employed to form the IXcoating 220 may be: (1) purchased commercially from ResinTech, Inc., ofWest Berlin, N.J., (2) purchased commercially from the Dow ChemicalCompany of Midland, Mich., under the trademark Dowex® or Amberlite®, or(3) manufactured in accordance with Eyal, et al., “Nitrate-selectiveAnion-exchange Membranes,” J. Membrane Sci., vol. 38.2, pp. 101-111(1988).

In the embodiment of FIG. 2, the IX coating 220 covers the fullcircumference of the optical fiber 210. In alternative embodiments, theIX coating 220 covers less than the full circumference of the opticalfiber 210. In the IR sensor 200, the IR source 230 and IR detector 250are located proximate opposing ends of the optical fiber 210. Inalternative embodiments, the IR source 240 and IR detector 250 arelocated at the same end of the optical fiber 210, a mirror (not shown)is located proximate the opposing end of the optical fiber 210, and theIR light 240 propagates twice through the optical fiber 210. In theembodiment of FIG. 2, the optical fiber 210 serves as the sensingelement. In an alternative embodiment, a coated wafer, formed bydepositing an IR-transparent material (e.g., nanocrystalline diamond) ona silicon wafer, serves as the sensing element. Coated wafers arecommercially available, for example, from Diamond Materials GmbH ofFreiburg, Germany.

The IX coating 220 and optical fiber 210 are placed in a subject liquid260, causing the subject liquid 260 to surround the IX coating 220.Consequently, the IX coating 220 absorbs at least some of any NO₃contained in the subject liquid 260 and transports the NO₃ toward theoptical fiber 210. An evanescent field projects from the surface of theoptical fiber 210 and interacts with the transported or absorbed NO₃.The IX coating 220 prevents fouling and interfering agents (such asparticles or other chemical components) from substantially interactingwith the evanescent field.

FIG. 3 schematically shows a cross-section of one embodiment of an IRsensor 300 employing an optical fiber 210 and a permselective membrane310 as the selective IX medium 100 of FIG. 1B. The IR sensor 300 hasmany elements in common with the IR sensor 200, so like referencenumerals designate like elements. However, in FIG. 3, a gap existsbetween the optical fiber 210 and the permselective membrane 310,creating a volume 320. The permselective membrane 310 absorbs at leastsome of any NO₃ contained in the subject liquid 260 and transports theNO₃ into the volume 320. The permselective membrane 310 prevents foulingand interfering agents from substantially interacting with theevanescent field.

The IR sensor 200 of FIG. 2, with its IX coating 220 is a simpler designthan the IR sensor 300, because the IX coating 220 directly covers theoptical fiber 210, and its performance is well-defined. However, theresin composing the IX coating 220 may require occasional regeneration.Further, continuous measurement of NO₃ concentrations when NO₃concentrations decrease over time typically requires the establishmentof a proper reference. A sub-cell (not shown) may need to exist in theIR sensor 200 to provide the measurements required to establish areference. This approach is widely used in gas sensors that employnon-dispersive IR (NDIR) detection, which will be described below. In analternative embodiment, the optical fiber 210 with its IX coating 220may be considered a disposable and replaceable element. This alternativeembodiment may be particularly advantageous for a portable or hand-heldwater or soil sensor.

The IR sensor 300 of FIG. 3, with its permselective membrane 310 is amore complex design than the IR sensor 200, because it requires theadditional volume 320. The permselective membrane 310 has at least twoembodiments for NO₃ transport: (1) electrochemical and (2) transport bymeans of Donnan dialysis, which uses counter diffusion of two or moreions through an IX membrane to achieve separation. Electrochemicaltransport requires external power, while Donnan dialysis does not.Donnan dialysis is therefore preferable for applications requiringcontinuous data collection by widely or remotely deployed IR sensors.

An alternative embodiment, which requires no additional illustration dueto its simplicity, dispenses with the selective IX medium, whether ittake the form of an IX coating as in the IR sensor 200 or apermselective membrane as in the IR sensor 300. Thus, the optical fiberis positioned in direct contact with the subject liquid. While thisembodiment is vulnerable to interference from inorganic and organicsubstances that absorb light at wavelengths similar to those of nitrate,one or both of an optical filter or spectrometer may be employed todefine a specific spectral range of operation. If a spectrometer isemployed, the principles of detection are, of course, spectroscopicrather than NDIR.

FIG. 4 schematically shows IR detection using the IR sensor. Moreparticularly, FIG. 1D shows a region (which may be the anion-exchangemembrane 220 of FIG. 2 or the volume 320 of FIG. 3) proximate theoptical fiber 110 and an evanescent field 410 created as the IR light240 propagates through the optical fiber 210 and reflects off itsboundary layer (not shown). (An evanescent field is sometimes referredto as a “leaky mode” or “tunneling waves.”) At least some of the NO₃ inthe region 220, 320 lies within the evanescent field 410. The NO₃interacts with the evanescent field 410, absorbing some of it andthereby changing the optical characteristics of the IR light 240propagating through the optical fiber 210. The change is a function ofthe concentration of the NO₃. The IR detector 250 is configured todetect the changes in the optical characteristics and produce anelectrical signal indicative thereof.

The theory of operation of the IR sensor is based on IR fiber-opticevanescent field sensing (FEWS) (Katzir, et al., “IR Fiber-OpticEvanescent Wave Spectroscopy (FEWS) for Sensing Applications (ConferencePresentation),” In Proc. SPIE 9703; p. 970308, and, Raichlin, et al.“Flattened Infrared Fiber-Optic Sensors for the Analysis of Microgramsof Insoluble Solid Particles in Solution or in a Dry State,” Vib.Spectrosc. 2014, vol. 73, pp. 67-72).

FEWS is similar to attenuated total reflection (ATR) sensing, which iswidely used. In ATR, light is totally internally reflected within asensing element. In the presence of a medium above the sensor surface(such as the anion-exchange membrane 220 of FIG. 2 or the volume 320 ofFIG. 3), the evanescent field is coupled into this medium with apenetration depth of a few micrometers, depending on the refractiveindex of the materials involved, operating wavelength and the angle ofincidence of the light forming the evanescent field.

The selective IX medium performs several important functions in the IRsensor 100. First, the selective IX medium substantially preventsfouling (Etheridge, et al., “Addressing the Fouling of In SituUltraviolet-Visual Spectrometers Used to Continuously Monitor WaterQuality in Brackish Tidal Marsh Waters,” J. Environ. Qual., vol. 42, p.1896, (1986)) caused by biological growth or chemical precipitation inthe context of environmental waters and agricultural soils, particularlyover long deployment periods. The film/resin/membrane in this casefunctions as a barrier that separates any fouling agents away from thesensing element, protecting the light propagating through it frominteracting with them.

Second, by preferentially absorbing/transporting NO₃ ions, the selectiveIX medium filters out inorganic and organic substances havingoverlapping absorption bands. This is useful, because it enables NDIRdetection (Wong, et al., “Non-Dispersive Infrared Gas Measurement,” IFSAPublishing (2012), and, Wong, et al., “Recalibration Technique for NDIRGas Sensors without the Need for Gas Standards,” Sens. Rev., vol. 32,pp. 217-221 (2012)) and avoids the need for a costly and bulky IRspectrometer. Preliminary results obtained from soil extract samplesindicate a unique NO₃ signature in the IR spectral range having a peakat about 1342 cm⁻¹. One particularly advantageous aspect of the IRsensor described herein is that this peak lies away from the absorptionbands caused by unwanted inorganic and organic substances.

NDIR is well-accepted and extensively used in low-cost commercial gassensors (e.g., Wong, et al., supra). A typical NDIR gas sensor has an IRsource, a sample chamber and an IR detector, and often passes the IRlight through a wavelength-selective (gas-selective) optical filter. Thesample chamber is filled with a gas, and the IR light passes from the IRsource, through the sample chamber and to the IR detector, whichproduces a signal that is processed to indicate absorbed intensity. Thewell-known Beer-Lambert law (or Beer's law) may then be used tocalculate the concentration of the gas from the absorbed intensity.However, aqueous environments present significant IR absorptionchallenges, which, to date, have prevented NDIR from being used in soiland water applications.

Conventional NDIR can be adapted in a nonobvious way to permit its usein aqueous environments. FIGS. 5A-5D show an NDIR-type sensor adaptedfor water and soil applications and associated filter meshes for thesoil application.

FIG. 5 is a cross-sectional view of one embodiment of an IR sensor 500configured for analyzing subject liquid. As with the IR sensor 300 ofFIG. 3, the IR sensor 500 has many elements in common with the IR sensor200 of FIG. 2, so like reference numerals designate like elements. TheIR sensor 500 has an outer case 510, which contains the subject liquid260 that surrounds the optical fiber 210 and the anion-exchange membrane220. The anion-exchange membrane may be an IX coating as in the IRsensor 200 of FIG. 2 or a permselective membrane as in the IR sensor 300of FIG. 3. The outer case 510 has an inlet 520 and an outlet 530 toaccommodate subject liquid flow into and out of the outer case 510. Anend cap 540 surrounds and protects the IR source 230 and, in someembodiments, has a mirrored inner surface (not separately referenced)allowing it to function as a reflector. In one specific embodiment, themirrored inner surface of the end cap 540 has a parabolic shape, and theIR source 230 is located at least proximate the focus of the mirroredinner surface. As a result, IR light emitted by the IR source in anydirection other than toward the end of the optical fiber 210 isreflected toward the end of the optical fiber 210. In some embodiments,a wavelength-selective optical filter 550 interposes an end of theoptical fiber 210 and the IR detector 250 to select the wavelength thatyields the most accurate data regarding the concentration of NO₃ anions(or any other nutrient, contaminant or combination thereof that aparticular IR sensor 500 may be configured to detect).

FIG. 6 is a cross-sectional view of one embodiment of an IR sensor 600configured for analyzing soil. As with the IR sensor 500 of FIG. 5 andthe IR sensor 300 of FIG. 3, the IR sensor 600 has many elements incommon with the IR sensor 200 of FIG. 2, so like reference numeralsdesignate like elements. In fact, the only difference between theillustrated embodiment of FIG. 6. and that of FIG. 5 is the addition ofat least one cylindrical filter mesh 610 surrounding the optical fiber210 and the anion-exchange membrane 220. The cylindrical filter mesh 610is configured to reduce the number of soil particles that can contactand damage the anion-exchange membrane 220.

Some embodiments of the IR sensor 600 include multiple, concentriccylindrical filter meshes. FIG. 7A is a partial isometric view of oneembodiment of concentric cylindrical filter meshes 710, 720 forfiltering soil. FIG. 7B is an isometric view of the concentriccylindrical filter meshes 710, 720 installed in soil (unreferenced).FIG. 7B also shows the outer case 510 of the IR sensor 600 andunreferenced soil located between the cylindrical filter mesh 720 andthe outer case 510.

To demonstrate the feasibility of the proposed approach, preliminarytests were performed on water and soil samples using a “single bounce”test apparatus as illustrated in FIG. 8. The apparatus employed anattenuated total reflectance (ATR) element (specifically a Smart iTR™ATR sampling accessory commercially available from Thermo FisherScientific, Inc., of Waltham, Mass.) as a sensing element 210. Thesensing element 810 was built into a Fourier transform IR (FTIR)spectrometer (specifically an iS™ 50R FTIR spectrometer alsocommercially available from the aforementioned Thermo FisherScientific), which served as an IR detector (not shown). Both ATR andFEWS employ evanescent field sensing, therefore they were regarded asequivalent for purposes of these tests. A sample (an aqueous solution,or IX medium) 820 was placed adjacent an unreferenced reflection surfaceof the sensing element 810, and within an evanescent field 830 generatedwhen IR light 840 was caused to propagate through and reflect off thereflection surface of the sensing element 810.

FIGS. 9A and 9B show data obtained from nitrate standard solutions(commercially available from Sigma-Aldrich Corporation, of St. Louis,Mo.) at concentrations of 12.5 ppm (a curve 910), 25 ppm (a curve 920),50 ppm (a curve 930), 100 ppm (a curve 940) and 200 ppm (a curve 950). Acurve 960 shows data obtained from a soil-extracted solution(specifically garden soil) for comparison. FIG. 9B demonstrates a lineardependence between NO₃—N concentration and peak area. Independentinvestigation of NO₃—N concentration in this sample of thesoil-extracted solution by a colorimetric technique was found to agreewith the FTIR data, showing a concentration of 25 ppm.

The data in FIG. 9A demonstrates that the detection of 12.5 ppm ofnitrate (the curve 910) is by no means the limit, and the detectionsensitivity can be well in the single-digit ppm range. A multi-bouncetest apparatus is expected to have a greater sensitivity.

While the well-defined nitrate absorption band obtained from the soilsample of the curve 960 seems to justify foregoing a selective IXmedium, the problem of interfering absorption bands from inorganic andorganic substances that absorb light at wavelengths similar to those ofnitrate becomes more prominent when the experiments are carried out inthe field, both in soil and in environmental waters. In fieldapplications one needs to take into account the attenuation of IR lightradiation due to fouling that comes from suspended particles andchemical precipitations. This attenuation may be mistakenly interpretedas resulting from NO₃ absorption. Therefore, a selective IX medium isimportant in applications for which spectroscopic detection isimpractical.

FIG. 10 shows data obtained from Amberlite IRA-400(CL) IX resin soakedin NO₃ at concentrations of 100 ppm (a curve 1010) and 200 ppm (a curve1020) for 30 min. After that, the resin was washed in water (without anydetectable nitrate levels) for 10 min. For comparison, data obtainedfrom resin soaked in water is also shown (a curve 1030), where the peakdue to the NO₃ is absent.

All data shown in FIG. 10 was referenced to data obtained from resinpre-soaked in water for 10 min. A dashed spectrum (a curve 1040) shows acomparison to the data obtained from 200 ppm NO₃—N dissolved in anaqueous solution. The spectrum was plotted with a vertical shift forclarity. The net absorbance value due to the NO₃ appears to be strongerin resin than in the water, which is merely the effect of thepenetration depth difference inherent in pressed resin particles versusaqueous drops. The NO₃ peak appears to be sharper in the resin, with thepeak at 1385 cm−1 visible as a shoulder. This is due to an overlap withthe disappearing (diminishing) absorption band due to ion exchange. Inthe proposed instrument, this effect can be treated using an appropriatecalibration procedure and algorithms. This illustrates the importance ofcalibration routines, which should be applied separately to eachmaterial when designing a particular sensor. In addition, the form ofthe resin (film, membrane or particle) and its contact with the sensorare important, as it dictates the strength of the obtained peak.

FIGS. 11A-11D show time-series data obtained from NO₃ at concentrationsof 12 ppm (FIG. 11A), 25 ppm (FIG. 11B), 50 ppm (FIG. 11C) and 100 ppm(FIG. 11D) absorbed in a membrane over intervals of 10 min. (curves1110), 60 min. (curves 1120), 120 min. (curves 1130), 140 min. (curves1140) and 180 min. (curves 1150). For these tests, the membrane wasimmersed into an NO₃ solution for specified periods of time, thenthoroughly washed in water to prevent any excess drops of NO₃ solutionon the sensor. The ion exchange within the membrane is evident from thegrowth of the peak in all of the presented plots. FIG. 12 summarizes theareas of the peaks as a function of time for concentrations of 12.5 ppm(a curve 1210), 25 ppm (a curve 1220), 50 ppm (a curve 1230) and 100 ppm(a curve 1240).

FIG. 13 is a flow diagram of one embodiment of a method of detectingnutrients and contaminants in agricultural soils or environmentalwaters. The method begins in a start step 1310. In a step 1320, IR lightis emitted by an IR source into a sensing element. In a step 1330, theIR light is propagated through the sensing element. The propagatinggenerates an evanescent field about the sensing element, and molecules(e.g., ions) in a subject liquid at least proximate the sensing elementinteract with and affect the evanescent field and thereby affect acharacteristic of the IR light propagating through the sensing element.In a step 1340, the characteristic is detected. In a step 1350, thecharacteristic is analyzed to determine a concentration of molecularspecies (e.g., ions) in the subject liquid. The method ends in an endstep 1360.

Those skilled in the art to which this application relates willappreciate that other and further additions, deletions, substitutionsand modifications may be made to the described embodiments.

What is claimed is:
 1. A method of operating an infrared (IR) sensor,comprising: generating IR light from an IR light source; receiving in anoptical fiber the IR light from the IR light source, wherein a selectiveion-exchange (IX) medium is associated with the optical fiber and the IRlight generates an evanescent field about the optical fiber as the IRlight propagates therethrough, the selective IX medium configured totransport an ion species in a subject liquid about the optical fiber;and receiving in an IR light detector the IR light from the opticalfiber, wherein the ion species affects the evanescent field and therebya characteristic of the IR light.
 2. The method as recited in claim 1wherein the selective IX medium is selected from the group consistingof: an IX resin coating, a permselective membrane, and an ion-exchangefilm.
 3. The method as recited in claim 2 wherein the IR detector isselected from the group consisting of: a thermal detector, and aphotonic detector.
 4. The method as recited in claim 2, wherein at leastthe optical fiber is surrounded by an outer case.
 5. The method asrecited in claim 4, wherein at least one filter mesh is located withinthe outer case and surrounds the sensing element.
 6. The IR sensor asrecited in claim 2, wherein a wavelength-selective optical filterinterposes the optical fiber and the IR detector.